The origin of slow PIII in frog retina: current source density analysis in the eyecup and isolated retina

The objective of this research was to determine the sources and sinks of current underlying the slow PIII component of the electroretinogram. Current source density analysis of the ERG evoked by diffuse light flashes was performed in eyecup and isolated retinas of frog. Blockade of synaptic transmission with aminophosphonobutyric + kynurenic acids simplified the CSD profiles through the retina. In addition to the photoreceptor source/sink pair, there was evidence for a major slow PIII source near the outer limiting membrane, a major sink near the inner limiting membrane, and a small source near the inner plexiform layer. Addition of Ba2+ abolished the slow PIII source/sinks, and it left only the photoreceptor source and sink. The results support the idea that slow PIII originates through K+ spatial buffering by Müller cells. Specifically, the light-evoked decrease in [K+]o in the subretinal space causes a primary K+ efflux from Müller cells (current source) and a primary K+ influx at the Müller cell endfeet (current sink). A decrease in [K+]zero in the proximal retina, caused by diffusion of K+ to the subretinal space, results in K+ efflux (the current source) at the inner plexiform layer.